mobiai-kmp

Kotlin Multiplatform (KMP)

Community contribution welcome! Help flesh out this skill with KMP patterns, common pitfalls, and real-world workflows.

Guide for working with Kotlin Multiplatform projects.

When to Use

• Working on shared Kotlin code that targets multiple platforms
• Debugging platform-specific issues in KMP projects
• Writing tests for shared modules

Detecting a KMP Project

# Check for KMP indicators
ls shared/ composeApp/ iosApp/ 2>/dev/null
grep -r "kotlin(\"multiplatform\")\|KotlinMultiplatform" build.gradle* --include="*.gradle*" | head -5
cat shared/build.gradle.kts | head -30

Project Structure

project/
  shared/                          # Shared KMP module
    src/
      commonMain/kotlin/           # Shared code (all platforms)
      commonTest/kotlin/           # Shared tests
      androidMain/kotlin/          # Android-specific implementations
      iosMain/kotlin/              # iOS-specific implementations
    build.gradle.kts
  composeApp/                      # Android app (Compose)
    src/main/
  iosApp/                          # iOS app (SwiftUI/UIKit)
    iosApp.xcodeproj
  build.gradle.kts                 # Root build file

Key Concepts

expect/actual

Shared interface with platform-specific implementations:

// commonMain
expect class PlatformLogger() {
    fun log(message: String)
}

// androidMain
actual class PlatformLogger {
    actual fun log(message: String) = Log.d("KMP", message)
}

// iosMain
actual class PlatformLogger {
    actual fun log(message: String) = NSLog(message)
}

Finding expect/actual declarations

# Find expect declarations
grep -r "expect " shared/src/commonMain --include="*.kt" -l

# Find actual implementations
grep -r "actual " shared/src/androidMain --include="*.kt" -l
grep -r "actual " shared/src/iosMain --include="*.kt" -l

Build Commands

# Build shared module
./gradlew :shared:build

# Run common tests
./gradlew :shared:testDebugUnitTest

# Run iOS tests (on macOS)
./gradlew :shared:iosSimulatorArm64Test

# Build Android app
./gradlew :composeApp:assembleDebug

# Build iOS framework
./gradlew :shared:linkDebugFrameworkIosSimulatorArm64

Testing

Shared Tests (commonTest)

// shared/src/commonTest/kotlin/MyTest.kt
class MyTest {
    @Test
    fun testSharedLogic() {
        val result = calculate(2, 3)
        assertEquals(5, result)
    }
}

Platform-Specific Tests

// shared/src/androidTest/kotlin/AndroidSpecificTest.kt
class AndroidSpecificTest {
    @Test
    fun testAndroidImplementation() { ... }
}

Common Issues

• iOS build fails: Check that the shared framework is linked correctly in Xcode
• expect/actual mismatch: Ensure all expect declarations have actual implementations for all targets
• Dependency conflicts: Use api() vs implementation() carefully in shared module
• Kotlin/Native memory model: Be aware of the new memory model (default since Kotlin 1.7.20)

Tracing Bugs

1. Determine if the bug is in shared or platform code — check the stack trace
2. If shared: fix in commonMain, test in commonTest
3. If platform-specific: fix in androidMain/iosMain, test in platform-specific test source set
4. If the bug manifests differently per platform: check expect/actual implementations

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Want to improve this skill? Share your KMP expertise, Compose Multiplatform patterns, and platform bridging tips via a PR.